Aircraft: B757
Phase of flight: Cruise
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy

Just before descent; the 'A' Flight Attendant advised me that [a few] passengers complained that their seat belts were too short. They were both frequent flyers and said they knew the belts were shorter than usual. I was concerned that perhaps the belts had been caught under the seat cushions during a security sweep. I asked the 'A' Flight Attendant to ask them to pull hard on the belts to be sure they were fully extended. I also told her that if we ended up with a planned emergency landing for any reason to move them to empty seats in coach as a precaution. I was very impressed that the 'A' Flight Attendant had taken the issue seriously and forwarded the information to me.I wrote it up in the logbook and advised Maintenance Operations Center (MOC) via ACARS. After the passengers deplaned I checked the belts and found that all belts at [the first few rows] in first class were the same length. I pulled up four of the cushions at [these rows] and found all eight belts incorrectly installed - the attach hardware was facing aft instead of 45 degrees up/aft of forward. This made each belt about 4 inches shorter than it should be; and completely invalidated the function of the seat belt under loads that could be experienced in a survivable accident. The hardware was tightened in that position so that the belts could not be repositioned without tools. I had read in the AML that the aircraft had recently completed a C-Check. I suspect that the belts were recently installed on the seats; possibly by a Maintenance Contractor during that maintenance check. If this is the case; it indicates a lack of knowledge and failure to follow written instructions. The Technician who came to address the issue said that not only was the hardware incorrectly installed but also some of the belts were in the incorrect position. I assume this referred which belt (male or female) should be on the left or right side of the seats. On [one row] the female belt was on the right side; but on [other row] the female belt was on the left side. I was concerned that it was possible that all belts on the aircraft had been incorrectly installed; and I suggested to the Technician that all the belts should be checked; and Maintenance Management should be advised of the situation. I am concerned that some technicians and inspectors apparently do not understand the critical safety importance of an improperly installed seat belt. In a hard stop accident; the attach hardware could bend and/or fail before stopping the seat occupant. This has lead to serious injury and death in otherwise survivable accidents.I am concerned that our maintenance inspection process; at some point; failed to identify and correct this situation. Seat belts had been incorrectly installed on at least four seats on the aircraft; invalidating their design function. [Recommend] inspect all seat belts on Aircraft X; if this has not already been done and correct any discrepancies.Investigate to determine where the seat belts were installed [at] and why inspection; after the work was not done or not done effectively. Develop and implement any changes in our maintenance and inspection programs necessary to prevent this issue from happening again. Share the general results of this investigation with the FAA and industry. If it can happen to us; it can happen to others. Identify any other aircraft in the fleet that could be similarly affected and inspect them immediately.Develop any necessary training and job aids appropriate to the duties of security sweepers; technicians; inspectors; flight attendants and pilots to address the serious safety consequences of improperly installed or positioned seat belts. I believe that it is particularly important to target the security sweep process; as this provides an opportunity to see any discrepancies on a daily basis. It may be necessary to designate a 'team lead' person trained to inspect the belts before securing the seat cushions. I consider this a massive failure of maintenance procedure and regulatory compliance; and a serious safety concern. Reporter stated seat belts can be inadvertently turned around when the seat cushions are removed and reinstalled during cabin security checks. He did not know the status or condition of the other remaining seatbelts on the B757-200 aircraft; but given what he was shown he suspects many of the 190 seats onboard most likely had similar issues. A spacer was missing on several of the seatbelt hardware that attaches to the seat frame. The spacer allows the seatbelt attachment to swivel and adjust to different size passengers. His company has not yet responded to his concerns. He has been flying the B757s for twenty plus years and has Aircraft and Powerplant (A/P) Certificates. A Captain was informed by a Lead Flight Attendant that passengers complained their seat belts were too short on a B757-200 aircraft. After landing; company Technician noted that not only was the hardware the seat belts attach to incorrectly installed; but the seatbelts were also incorrectly positioned at the seats. Captain also raises concerns their maintenance inspection process; at some point; failed to identify and correct the discrepancies.